It is known to produce ultra low sulphur diesel fuels by first hydrodesulphurising a hydrocarbon distillate stream boiling in the gasoil boiling range and then catalytically dewaxing the desulphurised distillate stream. The catalytic dewaxing step is needed for removing waxy molecules from the distillate stream in order to reduce the cloud point and the pour point of the gasoil. The desulphurised and dewaxed gasoil may be hydrofinished for saturation of aromatic compounds. The resulting desulphurised, dewaxed and optionally hydrofinished gasoil is then used as diesel fuel or diesel fuel component.
Also in the manufacture of lubricating oils, a dewaxing step is carried out for reducing the pour point of the resulting lubricating oil.
Catalytic dewaxing steps are usually carried out in a first stage dewaxing process or a second stage dewaxing process.
In a so-called first stage dewaxing process, a gas oil is first subjected to one or more hydrotreating steps which are followed by a first stage dewaxing step which as such can be followed by a further hydrotreatment step. In such a first stage dewaxing step use is made of a base metal catalyst such as nickel-containing catalyst on a medium pore zeolite support such as, for example ZSM-5.
In a second stage dewaxing process, the gasoil is first subjected to one or more hydrotreating steps followed by a second stage dewaxing step which is carried out using a noble metal-based catalyst on a medium or large pore zeolite. Most of todays' catalytic dewaxing processes are carried out in a first stage dewaxing mode of operation. In the hydrotreating step of such a first stage dewaxing process a portion of the heteroatom species is removed from the gas oil and aromatics are saturated. The effluent so obtained is then dewaxed in the dewaxing step, aromatics and heteroatom species that are still present in the dewaxed gas oil can be removed by means of a subsequent hydrotreating step. The drawback of such a process configuration, also when all three process steps are carried out in a stacked bed configuration, is that the hydrotreating steps are exothermic, whereas quite often the dewaxing step is endothermic, resulting in a challenging temperature control over the three catalyst beds.
Object of the present invention is to provide an improved hydrotreating and dewaxing process.